The invention relates to a transmitting optical element that can be used in a projection exposure apparatus for microlithography. Furthermore, the invention relates to a projection exposure apparatus for microlithography.
Projection exposure apparatus for microlithography are used in the production of semiconductor components and other finely-structured components. Apart from a light source and an illumination system for illuminating a photo mask or reticle, such a projection exposure apparatus includes a projection lens that projects the pattern of the reticle onto a light-sensitive substrate, for example a silicon wafer coated with a photo resist.
In order to produce ever smaller structures in the order of magnitude of less than 100 nm, up to now predominantly three approaches have been pursued. Firstly, attempts have been made to continue to enlarge the numerical aperture NA, on the image side, of the projection lenses. Secondly, the wavelength of the illumination light is continually being reduced, preferably to UV wavelength, in particular to wavelengths below 250 nm, for example 248 nm or 193 nm. Thirdly, further measures to improve the resolution are used, for example phase-shifting masks, multipolar illumination or oblique illumination.
Immersion lithography represents another approach to increasing the resolution. In this technique an immersion fluid is placed in the gap that remains between the last optical element on the image side, in particular a lens, of the projection lens and the photo resist or some other light-sensitive coating to be exposed. Projection lenses that are designed for immersion operation are also referred to as immersion lenses.
The advantages of immersion lithography are due to the fact that, as a result of the higher refractive index of the immersion fluid when compared to the vacuum, the exposure wavelength is reduced to an effective exposure wavelength. This is accompanied by an increase in the resolution and the focal depth.
The use of immersion fluids with a high refractive index makes it possible to achieve significant increases in the angle of incidence into the resist when compared to systems without immersion. However, in order to utilize the advantage of highly refractive immersion fluids to the maximum, it is necessary for the last optical element of the projection lens, which element is in contact with the immersion fluid, to also have a high refractive index.
In the case of UV wavelengths, in particular wavelengths below 250 nm, either silica glass or monocrystalline materials, for example calcium fluoride (CaF2), are used as materials for optical elements in a lens of a projection exposure apparatus for microlithography. At a wavelength of 193 nm, the refractive index of silica glass is 1.5603.
In monocrystalline materials with a cubic crystal structure, for example CaF2, the effect of intrinsic birefringence is noticeable in this wavelength range, and even more so in shorter operating wavelengths such as 157 nm. The dependence of the refractive index on the polarization state of the incident light, which dependence is caused by the intrinsic birefringence, limits the image quality of the projection lenses produced with these materials. For this reason, elaborate compensation measures are necessary, for example special lens designs with combinations of various birefringent lens materials or crystal orientation is required in order to ensure sufficient imaging quality of such projection lenses.
In John H. Burnett et al., “High Index Materials for 193 nm and 157 nm Immersion Lithography”, International Symposium on Immersion & 157 nm Lithography, Vancouver, Feb. 8, 2004, materials for application in a projection lens for microlithography, in particular in an immersion lens, are stated, among them alkaline earth metal oxide monocrystals such as MgO, CaO, SrO or BaO, as well as mixed crystals such as MgAl2O4 or MgxCa1-xO. However, already at 193 nm, all these materials show significant intrinsic birefringence. Thus there are problems that are very similar to those experienced with the use of CaF2.
WO 2006/061225 A1 therefore proposed the use of optical elements made of highly refractive polycrystalline material, for example polycrystalline spinel, such as magnesium spinel MgAl2O4, or polycrystalline garnet, such as yttrium-aluminum-garnet Y3Al5O12 or lutetium-aluminum garnet Lu3Al5O12, in a projection exposure apparatus for microlithography. Due to the statistical alignment of the crystal axes of the individual crystalline units, also referred to as crystallites, in a polycrystalline solid body the average value of the intrinsic birefringence in all spatial directions approaches zero. There is thus no need to provide complicated devices for compensating for the intrinsic birefringence. Magnesium spinel and the garnets stated in WO 2006/061225 A1 have high refractive indices of more than 1.8 at a wavelength of 193 nm, and are therefore particularly suited to immersion lithography.
From the literature, a band gap of 9 eV for magnesium spinel is known, for example from the article by J. D. Woosley, C. Wood, E. Sonder, and R. A. Weeks, “Photoelectric Effects in Magnesium Aluminum Spinel”, Phys. Rev. B, vol. 22, page 1065 (1980). This equates to a theoretical absorption edge of approximately 140 nm. However, more recent measurements have shown that the band gap of magnesium spinel is in fact somewhat smaller, namely only 7.8 eV. Accordingly, the absorption edge is around 160 nm. In the case of Lu3Al5O12 the band gap is even as low as 6.8 eV, which corresponds to an absorption edge of around 180 nm. When these materials are used in a projection exposure apparatus for microlithography with an operating wavelength of 193 nm, this operating wavelength is thus already very close to the absorption edge. Even a slight shift in the absorption edge towards higher energies can thus cause a significant deterioration of the transmission at this wavelength.